Data communication refers to the transmission of information over a medium. The medium, for instance, may be air (e.g., wireless or radio communications systems), fiber (e.g., fiber optic communications networks), or copper (e.g., wire conductor communication systems). The data communicated may be either digital (including pure digital and an analog representation of a digital code) or analog. The present invention is particularly directed toward a method and apparatus for correcting for signal degradation over a wire connection between a transmitting and receiving station of a digital data communications system. However, the invention is broader in concept and can be applied to various other types of data communication systems.
Data transmission over wire involves a transmitter station connected to a receiver station via a link (copper cable) of arbitrary length. Typically, digital information is transmitted as a series of pulses. One typical digital communications scheme is to represent a digital one with a pulse of a specified duration and specified amplitude and a digital zero with the absence of such a pulse in a designated time slot. An exemplary system might use a 3 millisecond pulse of 1 volt and/or -1 volt amplitude to represent a digital one. A digital zero might be represented by 0 volts for the same duration.
The front end of the receiver station includes apparatus, such as a phase-locked loop, for synchronizing to the data rate and phase of the incoming signal. Thus, the receiver can synchronize its clock to the incoming data so as to detect the signal level at proper instances relative to the time slot assignments of the incoming data signal. The receiver station, of course, typically also includes apparatus for decoding the incoming digital signal and using the received data for its intended purpose.
In order to maximize data throughput (the amount of information that can be sent in a given time period) and hence achieve faster data transmission speed, the pulses are packed as tightly as possible, i.e., the durations of the pulses are made shorter and the rising and falling edges are made sharper.
As is well known, the rising and falling edges of a pulse comprise a plurality of integer multiples of a base frequency sine wave of the same amplitude. The sharper the edge, the larger the number of frequencies comprising the signal. Therefore, as the rising and falling edges become sharper, the signal contains higher and higher frequency components.
However, as the frequency of a signal increases, the depth of penetration of the signal into the actual conductor, e.g., the copper, decreases. Consequently, the resistance of who the cable is greater for higher frequency signals than for lower frequency signals. This characteristic of copper wire is termed "skin effect." Due to skin effect, the attenuation of signals transmitted over a distance on a copper wire increases linearly with the square root of the signal frequency. Further, phase response increases linearly with frequency. The variable attenuation and phase response dependent on frequency thus causes the different frequency components that make up the pulses to arrive at the receiver with different amplitudes due to the variable attenuation and different delay due to the phase variation. Accordingly, the frequency components of the signal arriving at the receiver do not accurately represent the digital signal that was originally transmitted by the transmitter station.
The result is distorted pulses which spread out. If the spreading is significant enough, it can cause adjacent symbols (pulses) to overlap and interfere with each other. This type of signal degradation and interference is known as intersymbol interference or ISI. If the degradation is significant enough, the pulses and, thus the information contained in the signal, cannot be deciphered by the receiver station.
Accordingly, various methods and apparatus have been devised for correcting such signal degradation. The process of correcting for ISI is termed equalization. Equalization involves passing the received signal through circuitry that compensates for the attenuation and phase distortion induced by the medium and, thus, in theory at least, restores the signal to its original, transmitted, form. This type of equalization is termed inverting the channel response because the goal of the equalization is to modify the received signal by the inverse of the attenuation and phase responses of the wire. Another known solution is to shape the incoming pulses to a known response, one that may still retain ISI, rather than completely inverting the channel response.
Transmission of data over a wire connection also involves what is known as ohmic losses. Particularly, any signal transmitted over a wire attenuates over distance. Ohmic loss affects all frequencies equally and is thus termed flat loss. The greater the distance, the greater the ohmic loss or attenuation. Correction for ohmic loss typically involves the use of gain control circuitry.
Another feature of wire data communications systems is the need to electrically isolate the receiver and the transmitter from each other since they may be at different ground potentials, particularly if they are geographically distant from each other. Accordingly, receiver stations and transmitter stations typically are isolated from the transmission medium, i.e., the wire, by some mechanism. The typical isolation mechanism is the use of an isolation transformer between each communication station (receiver or transmitter) and the wire. However, an undesirable side effect of an isolation transformer is it induces a low frequency pole that rejects the low frequency components of transmitted pulses. This effect is another form of signal degradation in the wire communications. The low frequency rejection characteristics of transformers also can cause direct loss of data. For instance, a long string of consecutive digital is essentially is a low frequency analog signal which could be rejected by the transformer. To mitigate this undesirable side effect of isolation transformers, it is possible to ensure that the transmitted pulses do not contain any low frequency components. However, some line codes do and will comprise some low frequency components. If a line code (transmit pulse sequence) does contain low frequency tones, the transformer causes a low frequency disturbance known as baseline wander or BLW on the transmit pulse sequence that depends on the transformer and transmit pattern. As its name implies, baseline wander is an effect by which the ground voltage potential of the wire drifts from the desired potential. Baseline wander occurs when the signal being sent over the wire is not DC balanced over time and is quite common in communications systems. Thus, receiver stations may further be equipped with circuitry that compensates for the low frequency loss induced by the transformer.
From the discussion above, it can be seen that there are at least three effects that contribute to signal degradation in a wire communication system, namely, ohmic loss (which include transmit level inaccuracy, ohmic losses from the cable as well as ohmic losses of connectors and is collectively termed flat loss), base line wander, BLW, and frequency dependent degradation comprising variable attenuation and variable phase variation. These effects are a function of one or more of the (1) cable length, (2) cable composition, (3) transmitter output level inaccuracy, (4) transformer type, and (5) other environmental conditions which can vary from link to link. Error correction or compensation techniques to battle these effects therefore should be adaptive in nature, i.e., self-correcting, since many of these factors can vary from one received data link to the next.
The concept of adaptive compensation is based on the fact that the communication protocol includes intermittent known signals that can be readily identified at the receiver station. The degraded signal which is actually received can be compared with the ideal version of the known signal. The difference between the two can be determined and compensation can be applied to the signal which would exactly correct for the differences between the two signals. For instance, it may be known when to expect a particular falling edge of a pulse. It is further known that when the falling edge of a pulse crosses 0.5 volts, it should reach 0 volts a certain time thereafter, e.g., four nanoseconds. If, for instance, it takes longer than four nanoseconds to reach 0 volts, then the received signal contains ISI degradation which should be corrected.
In addition to the above-discussed effects that contribute to signal degradation, integrated circuits such as the electronics that might be found in a receiver station, are fabricated in a manufacturing process that contains certain non-idealities. Put simply, each die manufactured by a given process is not exactly identical to each other die; they contain differences due to variations in the fabrication process that cannot be controlled ideally. These process non-idealities in the receiver station circuitry lead to DC offsets which are superimposed on the incoming signal. As its name implies, DC offset results in a flat offset of the receiver station on the incoming signal. For instance, an incoming signal at what is intended to be 0 volts can be recognized at a DC level of some other voltage. The offset can be significant enough to result in the misinterpretation of digital ones as zeroes and vice versa.
FIGS. 1, 2, and 3 illustrate various techniques that have been employed in the prior art to correct for one or more of the aforementioned signal degradation effects.
For instance, FIG. 1 shows a receiver front end employing one typical adaptive equalization scheme of the prior art. As shown, an input signal received over the wire network from the transmitter station is received at input terminal 12. A comparator 16 compares the input signal with a predetermined level or levels to determine whether the signal at any given instant (within a time slot) should be interpreted as a digital 1 or a digital 0. For instance, in a communications system in which a 0 is represented by a 0 volt level for a specified duration and a digital one is represented by a 1 volt or -1 volt pulse, the comparator may compare the signal with +0.5 and -0.5 volts in order to determine whether the symbol in a time slot should be interpreted as a digital 1 or a digital 0. A timing recovery circuit 18, which may comprise a phase locked loop (PLL), recovers the timing of the signal so that the receiver station can synchronize itself to the time slot timing of the pulses of the signal. The timing recovery circuit 18 outputs the detected receive signal to further circuitry (not shown) which processes and/or uses the data for whatever the intended purpose might be. For instance, if the receiver station is a facsimile machine on a telephone communications network, the circuitry subsequent the timing recovery circuitry will translate the received pulses into an image and print the image on a sheet of paper.
The equalization circuitry for shaping the incoming data in order to correct for frequency dependent degradation comprises an adaptive equalizer 14 interposed in the receive path before the comparator and a feedback loop including an error generator circuit 20 and circuit 22 for performing an algorithm whereby an error signal is extracted from the equalizer output. Typically, equalizer block 14 comprises a filter with adjustable poles and zeros. By adjusting the poles and zeros, one can optimize the transfer response of the equalizer for any medium and hence reduce ISI. The optimization procedure requires an algorithm, generally known as the least mean square (LMS) algorithm, whereby an error signal is extracted from the equalizer output by the error generator block 20 and fed to the least mean square algorithm performing circuit 22. The LMS circuit 22 then controls the equalizer relative to the extracted error signal.
For the optimization procedure, a gradient signal for each adaptive pole and zero is required. This requirement increases circuit complexity, power dissipation and area. Thus, a more simplified architecture is desired.
FIGS. 2 and 3 show two variations of a second general type of architecture for correcting for signal degradation.
In these architectures, the equalizer poles and zeros are pre-computed over a range of possible trajectories that cover the full range of potential cable variations. The concept of this design is to form a programmable equalizer with N possible transfer functions. A feedback circuit is used to select one of the N possible transfer functions in an adaptive fashion based on the characteristics of the signal received at input terminal 12. In operation, an error signal from the output of the equalizer 32 is generated and a minimization routine is used to minimize the error signal as a function of one of the N programmed transfer functions. Also included within the circuit is a technique for compensating for baseline wander.
Referring first to FIG. 2, the receive signal is input to the adaptive equalizer 32. The comparator 16 and timing recovery 18 circuits are essentially identical to those discussed above in connection with FIG. 1. A low pass filter 36 is coupled in a loop around the comparator 16 and a high pass filter 34 is added between the equalizer 32 and the comparator 16. A summing circuit 44 sums the output of the low pass filter 36 and the high pass filter 34 before forwarding it to the input of the comparator. The high pass filter 34 rejects the low frequency components of the transmit pulse sequence. This induces baseline wander as previously explained. The low pass filter 36 then recreates the low frequency components from the equalized output and the two components are summed together by summer circuit 44 to recreate the full signal. This is known as a quantized-feedback technique.
With respect to equalization, the output of the equalizer 32 is fed back to control the equalizer through a peak detection circuit 38. The peak detector 38 essentially is a comparator which compares the feedback signal with a reference signal, the reference signal being the expected peak level. For instance, in the example which we have been using, the peak level is 1 volt or -1 volt. The peak detection circuit 38 computes the peak value L.sub..infin. norm of the equalizer output, compares that value to an expected value, and generates an output signal to adjust the equalizer until the difference (i.e., the error signal) is minimized.
A second low pass filter 40 (LPFT) is used to average the error signal over several pulses of the input sequence.
One drawback of the system of FIG. 2 is the use of the analog low pass filter 40. Particularly, an analog low pass filter implements an integrator that is leaky (non-ideal). Further, an active integrator suffers from DC offset, which degrades the optimization algorithm. Peak detection implementations are less than optimum because they cannot segregate out flat loss. Other options for peak detector circuit 38 are peak tracking, average value tracking (L.sub.1 norm) or power estimation (L.sub.2 norm). However, these approaches also do not differentiate between ISI and flat loss (which is further complicated in the presence of baseline wander). Hence, optimal equalization cannot be achieved in the absence of an accurate transmit amplitude and a known channel flat loss. These requirements are seldom achievable.
There are two potential locations for placing the high pass and low pass filters 34 and 36, respectively. The first option is as shown in FIG. 2 and discussed above. FIG. 3 shows a second option. In the FIG. 2 embodiment, the high pass filter 34 is placed between the equalizer 32 and the comparator 16 and the low pass filter 36 is in a feedback loop around the comparator 16. The summing circuit is positioned before the comparator and within the feedback loop. The low pass filter 36 and the high pass filter 34 typically are implemented such that they have the same pole frequency (i.e., the same time constant). The pole frequency typically is selected to be the same or higher as the low frequency pole of the transformer of the receiver station (not shown). However, depending on the cable length, the effective pole frequency of the transmit and the receive transformer combination can vary. Consequently, typical quantized-feedback techniques are effective only over a limited cable length variation and, in fact, can deteriorate overall performance at cable lengths outside the range for which it is effective. Thus, optimum baseline wander correction is unattainable with typical non-adaptive quantized-feedback baseline wander correction circuit techniques.
A drawback of the system of FIG. 2 is that an effect of baseline wander is an increase in the possible range of the signal envelope. This requires that the equalizer 32 operate over a wider dynamic range. Since this is not always possible to accomplish, the incoming signal must be attenuated to decrease its dynamic range within a range that can be handled by the equalizer 32. This can result in performance degradation because the signal to noise ratio may increase as the signal power is decreased.
The FIG. 3 embodiment of this technique eliminates this problem. In this embodiment, the high pass filter is placed before the equalizer circuit 32 and the feedback loop of the low pass filter 36 includes the equalizer. The advantage of this embodiment is that the baseline wander effect is corrected by the combination of the high pass filter 34, low pass filter 36 and summer 44 before the signal passes through the equalizer 32. Thus, in this embodiment, the dynamic range of the signal is corrected and reduced prior to passing through the equalizer 32. However, a drawback of this embodiment is that the effectiveness of the DC restoration (the correction of baseline wander) is sensitive to channel flat loss, which is an unknown. Also, the quantized feedback in the low pass filter loop portion of the baseline wander correction scheme of either the FIG. 2 or FIG. 3 embodiment is a positive feedback system. Hence, it is possible for the system to become unstable.